1. Field of the Invention
This invention relates to methods and apparatus for ascertaining the volume of liquid in a container or tank, and more particularly to a system of gauging liquid volumetric quantities based upon the properties of a gas and is operable in any gravity environment, including zero gravity environment.
2. Background
On the Earth, gauging systems for measuring liquid and gas quantities in a tank invariably rely on the Earth's gravitational field to separate the gas and liquid phases in a consistent and predictable manner. This type of separation provides a flat, or predictable interface of the gas and liquid phases from which volumetric measurements can be made. Current state-of-the-art measurement instruments are designed to locate this separation interface by some measured or inferred change in physical property. Knowledge of the storage tank geometry is then used to calculate the respective volumes of gas and liquid.
In a zero-gravity or weightless environment, however, the gravitational or acceleration body forces are absent, which introduces a number of concerns due to the behavior of the individual phases, both dynamic and static, and the effect on contents gauging systems. These effects may include:                1. no stable, known positioning or interface; configurations can take multiple geometries and this can result in multiple random distribution of interface locations;        2. for certain designs, wetting of sensor surfaces disables measurement capabilities;        3. loss of calibration. A calibration loss in a zero-gravity environment may make the system in-operative since calibration is based upon a fixed, known geometry of the materials in the contained system;        4. point sensor systems may not provide a true picture of the phase locations unless multiple sensors are used and statistical approaches are utilized. Continuous, total body systems require multiple detectors uniformly distributed over the container surface; and        5. temperature and/or composition variations (stratification) can cause significant variations in measurement accuracy.        
Because of these effects, current methods for zero-gravity tank gauging are limited to single phase storage (either gas or supercritical) or rely on flow rate measurements into or out of the tank in the case of single phase storage, considerable weight is added to the structure since these systems must be designed for high pressure storage. Alternatively, pressure monitoring systems are used to estimate volume as a function of pressure with poor accuracy. These systems cannot function in the presence of a leak.
Use of inlet/outlet flow gauging measurement may not provide a reliable (accurate) estimate of the tanks liquid contents due to the following restrictions:
1. Two-phase flow. Most flow metering systems are designed to handle single phase systems. Two-phase metering designs are limited in application.
2. Turn-down (range limitations). The turndown range of most flow gauging systems is at best 20 to 1. Non-steady state applications of these instruments is significantly limited.
3. System leaks. Storage tank leaks would go undetected since no direct means are available for contained liquid volume measurement.
In the present invention, the contents in a two phase system including a compressible gas and an incompressible substance are measured independent of the shape of the container or tank and independent of temperature. In another important aspect of the present invention, the ratio of specific heat constants can be obtained in a relatively simple fashion. There are many chemical, manufacturing and mechanical applications where volume measurements are needed. These volumetric measurements are to ascertain the gas, liquid, and/or solid contents within a container. There are many instances where the current technology applications are not appropriate, such as in zero or low gravity applications, certain high-pressure applications, with toxic and/or explosive chemical applications, and with underground cavern storage facilities.
Current technologies include level height measurements, sonic measurements, capacitance measurements, and nuclear radiative measurements. All of these technologies rely on the fluid in a state of quiescent liquid level produced by the effects of gravity on the system. In zero or low gravity applications, the fluid may not be so constrained.
U.S. Pat. Nos. 3,237,451, 3,413,847 and 3,769,834 all disclose methods to determine the volume of gases in tanks in a zero gravity situation. In U.S. Pat. No. 3,237,451, an acoustic system is utilized for generating pressure changes which are related to volume measurements. In U.S. Pat. No. 3,413,847, density of a gas is measured and related to volume measurements. In U.S. Pat. No. 3,769,834, the volume of a human body is measured by changes in pressure.
U.S. Pat. No. 3,585,861 discloses a system for determining volume of gases by using a reference gas pressure. U.S. Pat. No. 4,072,050 utilizes thermal energy changes for measuring volume. U.S. Pat. No. 4,384,925 utilizes electrochemical sensing procedures for measuring volume. U.S. Pat. No. 4,726,216 relates to a calibration circuit for detecting “HC” gases. U.S. Pat. No. 4,781,061 relates temperature to volume measurements. Each of the above-identified are herein incorporated by reference in its entirety.
In 1 g applications, the gauging methods may be inappropriate due to the location of the container, such as an underground cavern or may not be able to be mechanically capable or safe for measuring certain fluids. Additionally, waves, bubbles, froth or foam may interfere with these measurements, and no device known is capable of measuring the gas contents in a vessel as a direct measurement.
Within the fluid-chemical process industries, there is a significant need to improve tank and vessel gauging capabilities. The bulk of current designs include differential pressure for liquid level, sonar, radar, load cells, radiation, capacitance, etc., which can be expensive and require extensive calibration and maintenance requirements. These systems have also proven to be unreliable or unsuitable for certain applications and are typically highly dependent on fluid properties and process conditions. In zero-G environments, where the solids or liquids may float within the containment volume, none of these systems works suitably or accurately.
Thus, the conventional methods and devices lack (1) the capability to get an accurate volume measurement, 2) the ability to accurately measure container contents without a definable phase interface, 3) the ability to be applied to different container volumetric requirements, shapes, and internal features, and 4) the ability to accurately measure and self-calibrate the volumetric equations for highly accurate metering capabilities.